1. Field of the Invention
The present invention relates to a process for producing a lithium-containing composite oxide for a positive electrode for a lithium secondary battery, which is low in gas generation and has a large volume capacity density, high safety, a high average operating voltage, excellent durability for charge and discharge cycles and excellent low temperature characteristics; a positive electrode for a lithium secondary battery containing the lithium-containing composite oxide produced; and a lithium secondary battery.
2. Discussion of Background
Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolyte secondary battery such as a lithium secondary battery which is small in size and light in weight and has a high energy density, has been increasingly high. As a cathode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
Among them, a lithium secondary battery using a lithium-containing composite oxide (LiCoO2) as a cathode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, can produce a high voltage at a level of 4V, whereby it has been widely used as a battery having a high energy density.
However, in the case of the non-aqueous type secondary battery using LiCoO2 as a cathode active material, further improvement of the capacity density per unit volume of a positive electrode layer and the safety, has been desired. On the other hand, there has been a problem of deterioration of the cyclic properties such as gradual reduction of the battery discharge capacity due to repetitive charge and discharge cycles, a problem of the weight capacity density or substantial reduction of the discharge capacity at a low temperature.
Further, recently, in addition to a cylindrical cell used for e.g. a laptop computer, a prismatic cell or an aluminum laminate prismatic cell has been widely used for e.g. a mobile phone which has been required to be thin. However, there is a serious problem that a prismatic cell is swelled by the increase of the internal pressure due to the gas generation, which has not been problematic in the case of the conventional cylindrical cell having a strong cell structure. The gas generation in the secondary battery is caused by thermal decomposition of a cathode active material itself or the decomposition of an electrolyte by its contact with a cathode active material. Therefore, it is desired to have a positive electrode material which can suppress gas generation more than ever.
In order to solve these problems, Patent Documents 1 to 3 have proposed to coat the surface of a cathode active material with e.g. Al2O3, AlPO4, TiO2 or ZrO2 which can inactivate the reaction with an electrolyte, but there has been a problem such that conductivity of cathode active material particles is lowered. Further, Patent Documents 4 and 5 have proposed to apply a lithium-titanium composite oxide as a coating material having conductivity imparted. However, in such a case, though the discharge capacity is improved at the time of high-current discharge, the effect of suppressing gas generation has not sufficiently been improved.
On the other hand, in order to prevent deterioration of cyclic properties and suppress gas generation, Patent Document 6 discloses a lithium composite oxide having a composition of LiBxCo(1-x)O2 (wherein 0.001≦x≦0.25) which is obtained in such a manner that when a cathode active material as a lithium-containing composite oxide is prepared, a boron compound is mixed with a precursor material, and the mixture is fired to replace a part of cobalt with boron.
Further, in order to improve heat stability, Patent Document 7 discloses a composite oxide comprising lithium and a transition metal, such as LixNiyCo0.2O2X(1-y-z)Ow (wherein X is Al or Mn, 0.95≦x≦1.10, 0.1≦y≦0.9, 0.1≦z≦0.9, 1.8≦w≦2.2) which is obtained in such a manner that a lithium compound and lithium borate are mixed with a precursor material for cathode active material, and the mixture is fired to let lithium borate be present on the surface of particles.
However, heretofore, a cathode active material made of a lithium-containing composite oxide has been unknown, which can remarkably suppress gas generation during its use, without impairing any one of characteristics such as volume capacity density, safety, average operation voltage, durability for charge and discharge cycles and low temperature characteristics.
Patent Document 1: JP-A-2003-234102
Patent Document 2: JP-A-2003-7299
Patent Document 3: JP-A-2003-221234
Patent Document 4: JP-A-2004-319105
Patent Document 5: JP-A-2004-103566
Patent Document 6: JP-A-5-325971
Patent Document 7: JP-A-2004-335278